The principle of communication by frequency hopping is to transmit data (information, video, speech, etc.) successively over a variety of frequency channels, pseudo-randomly.
In civilian applications, this technique can be introduced to deal in particular with two issues:
                Fading: radio signals are subject to fading, which is approximately distributed according to Rayleigh's law. Since Rayleigh-type fading is frequency-selective, frequency hopping coupled to the channel encoding and to interleaving makes it possible to average the risks of losing information. This improvement in the quality of the transmission link is all the more necessary when the nodes are moving.        Interference: without frequency hopping, the strong signals obtained from the adjacent cells continuously affect the communication. With random frequency hopping, the cells use pseudo-random hopping sequences, thus making the interference random.        
In some network applications, the whole of one and the same network knows a frequency hopping law. A node arriving late in the network is synchronized on the frequency changes and determines the progress in the pseudo-random sequence thanks to the knowledge of the hopping law. A node transmits within a frequency level. Each new level has a corresponding new frequency. A hold time is provided at the start of each level to enable the radio subsystem to synthesize the new frequency. This time is linked to the performance of the synthesizer and to the desired frequency accuracy. Typically, this time varies between 200 μs and a few ms. Certain communication system architectures implement two synthesizers to be faster.
In the Defence and Security domain, the frequency hopping mechanism is used to fight against scrambling and reinforce discretion.
To fight against multiple-paths, various techniques known to those skilled in the art can be used, for example, the techniques of equalization, spectrum-spreading and multiple carriers.
It is known from the prior art to use the orthogonal frequency-division multiplexing technique OFDM for frequency-hopping communications. FIGS. 1 and 2 represent a multiple-carrier modulation. The OFDM technique (Orthogonal Frequency Division Multiplexing) is a multiple-carrier modulation, that is, it consists of a set of orthogonal carriers. The orthogonality of the OFDM modulation is achieved by choosing harmonic frequencies of a base frequency and using them over a duration that is a multiple of the period of the base harmonic.
In the example given in FIG. 2A, 2B, 2C, 8 subcarriers are used (f0, 2f0, 3f0, 4f0, 5f0, 6f0, 7f0, 8f0). However, as a general rule, also used is the continuous component which is orthogonal with any sinusoid if the energy is integrated over a duration equal to or a multiple of a period of f0. In this case, the continuous component and the harmonics f0 to 7f0 are used. Each of these harmonics is modulated by a signal to be transmitted with a modulation chosen from the phase modulations (BPSK, QPSK, etc.) or from the amplitude modulations (16QAM, 64QAM, 256QAM, etc.). The more effective the modulations are in transmission density per Hz used, the greater is the range reduction.
A transmitted symbol is a set of binary information transmitted on the different carriers of the OFDM multiplex. Thus, in the example of FIG. 2A, 2B, 2C, the symbol comprises binary information on the eight carriers f0 to 8f0. Knowing that 1 bit, 2 bits, 4 bits, 6 bits, 8 bits are respectively deducted from a symbol in BPSK, QPSK, 16QAM, 64QAM and 256QAM modulations, all the information transmitted by a complete OFDM symbol is therefore, according to the abovementioned modulations, 8, 16, 32, 48 or 64 bits.
FIG. 3 diagrammatically represents an exemplary OFDMA sender (general case with several users within one and the same symbol). It comprises, for example, the following modules:                A device 1 for allocating subcarriers according to the users with different modulations,        Several “adaptive” modulation means, 2k, the number of these means is, for example, equal to the number of the users k,        A device 3 adapted to switch from the frequency domain to the time domain,        A device 4 for inserting a hold time or cyclical prefix to avoid overlaps of the different multiple-paths,        A device 5 P/S (Parallel to Serial)        A sending antenna 6.        
FIG. 4 represents an exemplary architecture for an OFDMA receiver. It comprises, for example, the following modules:                A receiving antenna 7, linked to an S/P device 8        device 9 adapted to delete the cyclical prefix introduced on sending,        A device 10 for switching from the time domain to the frequency domain,        A means 11 adapted to extract the subcarriers for each user k,        Several adaptive demodulation devices 12k.         
In certain standards, for example, HIPERLAN2 and IEEE802.16d, OFDM is used with a TDMA-type access protocol. The timeslot of the TDMA (Time division multiple access) cycle is then a whole number of OFDM symbols.
It is also possible to use OFDM to share access between several users by the sub-channelization technique, or with OFDMA (Orthogonal Frequency Division Multiple Access) where the N carriers are not allocated to a single user. The set of the N carriers is subdivided into M subsets of carriers. The resources are allocated subset by subset. The M users transmit concurrently, within the same OFDM symbol.
FIG. 5 diagrammatically represents a communication network architecture based on OFDMA. The latter requires an allocation of the resources between the different users. This allocation is dependent on the quality of service and the bit rate requested for each user and on the environment (response of the channel for each user, interference, etc.).
In the example of FIG. 5, K users communicate within one and the same OFDM symbol.
The identical modules in FIGS. 3, 4 and 5 bear the same references. In addition to the diagrams of FIGS. 3 and 4 respectively concerning a sender and a receiver, the OFDMA system notably comprises a resource allocation module 13 which receives the allocation requests from the different users, an indication concerning the maximum power needed, an indication concerning the channel of the user, and which delivers signals to the subcarrier allocation device, and to the device for extracting subcarriers for the different users, and to the adaptive demodulation modules.
Different resource allocation algorithms are available in the literature. These algorithms make it possible notably to allocate the subcarriers to the users and determine the modulation/coding type for each of these carriers. These algorithms are critical for making best use of OFDMA. In practice, the subcarriers that are “favorable” to a user, that is, those least affected by disturbances, of OFDMA may be unusable or less favorable to other users. The resource allocation algorithms are responsible for exploiting this diversity.
The FH-OFDMA (or Frequency Hopping OFDMA) concept is known from the prior art (Hikmet Sari 1997 “Orthogonal frequency-division multiple access with frequency hopping and diversity” in Multi-carrier Spread-Spectrum, K. Fazel and G. P. Fettweiss, Eds. Kluwer Academic Publishers, 1997, pp 57-68) in the case where the transmission channel is not known. Since a channel can contain carriers that are unusable because of interference, each user is assigned a sequence of carriers rather than a particular carrier. The sequence used is generally incremental.
Spectrum spreading is conventionally done by introducing at the radio level a synthesizer with fast tuning, of the order of 100 to 150 μs, to hop over a wide bandwidth.
Frequency hopping is applied to protect the synchronization of the nodes between themselves. In the case of a synchronization (time/frequency) using dedicated signals (case of the short and long preambles of 802.16), a synchronization protection may be implemented by a sequence of pseudo-randomly drawn signals. Thus, the synchronization patterns are not systematically the same, but vary, in addition, according to a sequence defined by pseudo-random drawing.